1. Field of the Invention
The present invention relates to anti-corrosion ceramics that can be favorably used as a member for high-temperature use and, particularly, as parts for heat engines, such as parts for gas turbine engines.
2. Description of the Related Art
On account of their excellent heat resistance, resistance against heat and shock, abrasion resistance and resistance against oxidation, efforts have been made to utilize silicon nitride, silicon carbide and Sialon which are known engineering ceramics as parts for the heat engines and, particularly, as parts of gas turbines and turbo rotors.
Such non-oxide ceramics containing silicon, to which are added a sintering assistant, are usually highly dense and are highly strong. In the case of silicon nitride ceramics, for example, there can be obtained a sintered body of silicon nitride by adding Y2O3, Al2O3 or MgO as a sintering assistant to a powder of silicon nitride followed by firing.
The above ceramics can be used in a temperature region which is as high as 1000xc2x0 C. or higher where the metal materials could not be used, and realizes a heat efficiency of, for example, not lower than 40% which could not be accomplished with the conventional metal materials.
When these ceramics are to be used as members for internal combustion engines and, particularly, for gas turbines, there are required not only the strength but also other properties under severe conditions of high temperatures. In the combustion engines such as gas turbine engines, the ceramics must exhibit a strong resistance against corrosion due to air streams of high temperatures and must further exhibit abrasion resistance and shock resistance against the collision by very fine particles.
The above-mentioned ceramics containing silicon exhibits excellent resistance against the corrosion due to the water vapor contained in the fuel gas and excellent abrasion resistance, but reacts with the water vapor of high temperatures contained in combustion gas from the gas turbine. Due to the corrosion caused by the water vapor of high temperatures, therefore, the ceramics are worn out vigorously arousing a problem of short life. In particular, such parts as a combustor liner, a transition duct and a stator blade used for the gas turbines are exposed to the combustion gas of high temperatures containing water vapor, and the ceramics on the surfaces are worn out conspicuously.
In order to solve the above problems, therefore, a variety of attempts have been made concerning the sintering assistant, grain boundary phase, firing conditions and forming protection film against oxidation. In order to improve resistance against the water vapor of high temperatures, for example, Japanese Unexamined Patent Publication (Kokai) No. 183676/1997 proposes coating the surfaces of a sintered body comprising chiefly silicon nitride or Sialon with a glass layer comprising chiefly SiO2.
Attempts have also been made to improve resistance against the oxidation, resistance against the erosion and resistance against the corrosion by forming a protection film of alumina or mullite having good resistance against the oxidation on the sintered body of silicon nitride by such a method as CVD or a melt injection.
According to the method of forming a glass layer on the surface as taught by Japanese Unexamined Patent Publication (Kokai) No. 183676/1997, however, the properties can be improved under static conditions where there is no air stream. When really exposed to the gas of high temperatures, high pressures and high speeds in the engine, however, the glass layer on the surface is rapidly worn out since the glass evaporates. Namely, the glass layer has a short life and fails to serve as a protection film.
Alumina and mullite exhibit higher resistance against the corrosion than the silicon nitride. In an atmosphere having a high water vapor partial pressure, however, the alumina and mullite exhibit small resistance against the corrosion and small durability, and are not satisfactory from the practical point of view.
It is therefore an object of the present invention to provide anti-corrosion ceramics having a high resistance against the corrosion caused by the water vapor of high temperatures and offering a long life in a high-temperature region of not lower than 1000xc2x0 C.
The present invention was accomplished based on a discovery that the resistance against the water vapor of high temperatures can be improved if a stabilized zirconia layer is formed on the surface of the silicon-containing ceramics and if its composition is controlled. The invention realizes ceramics that can be favorably used, particularly, as parts for constituting internal combustion engines such as gas turbine engines that operate at high temperatures.
According to the present invention, there is provided anti-corrosion ceramics comprising a substrate of at least one kind of silicon-containing ceramics selected from a silicon nitride, a silicon carbide and Sialon, and a surface protection layer formed on the surface of the substrate, wherein the surface protection layer comprises a zirconium oxide stabilized with an element of the Group IIIa of periodic table, and the total amount of Al and Si in the surface protection layer is suppressed to be not larger than 1% by mass.
In the surface layer of a zirconium oxide (hereinafter often referred to simply as stabilized zirconia) stabilized with an element of the Group IIIa of periodic table, the contents of Al and Si, that are subject to be vigorously worn out due to the reaction with the water vapor of high temperatures, have been suppressed to be small without exceeding a predetermined amount. As a result, the surface protection layer is strong against the corrosion due to the water vapor of high temperatures, and is capable of protecting the surfaces of the ceramics containing silicon. Accordingly, the anti-corrosion ceramics of the present invention is very suited for such applications, particularly, as parts for gas turbine engines.
In order to effectively prevent the surface layer from peeling due a difference in the thermal expansion from the substrate of silicon-containing ceramics, it is desired that the surface protection layer has a thickness of from 5 to 200 xcexcm.
It is further desired that the element of the Group IIIa of periodic table used for stabilizing the zirconium oxide is at least one selected from Y, Er, Yb and Lu. This is because when the zirconium oxide is stabilized with the element having a small ionic radius of the Group IIIa of periodic table as described above, the coefficient of thermal expansion becomes relatively small, making it possible to effectively prevent the surface protection layer from peeling due to the difference in the thermal expansion. It is desired that the element of the Group IIIa of periodic table is existing in the surface protection layer in an amount of from 3 to 15% by mol calculated as an oxide thereof. The surface protection layer formed of the zirconium oxide stabilized with the element of the Group IIIa of periodic table is peeled or worn out little even when it is exposed to the gas of a high temperature, a high pressure and a high speed, exhibiting markedly improved resistance against the oxidation, resistance against the erosion and resistance against the corrosion.
It is desired that the surface protection layer has a porosity of from 1 to 30% and, particularly, from 5 to 30%. This relaxes the stress due to a difference in the thermal expansion between the substrate and the surface protection layer, and suppresses the surface layer from peeling.
It is further desired that the surface protection layer has a texture extending like columns toward the surface from the interface to the substrate. This is because, even when the surface protection layer is cracked due to the thermal stress, cracks develop in the columnar interface and, hence, the surface protection layer does not peel off the substrate but stays adhered to the substrate.
In the present invention, an intermediate layer comprising RE2Si2O7 and/or RE2SiO5 (RE: element of the Group IIIa of periodic table) can be provided between the substrate and the surface protection layer. Provision of the intermediate layer works to relax the stress caused by the difference in the thermal expansion between the substrate and the surface protection layer, and to suppress the surface layer from peeling.
The intermediate layers may be formed in a plurality of numbers. In this case, it is desired that the intermediate layers are formed of disilicate crystals (RE2Si2O7) of different elements (RE) of the Group IIIa of periodic table, the intermediate layers being so arranged that the coefficients of thermal expansion of the intermediate layers gradually increase from the substrate toward the surface protection layer (which has a coefficient of thermal expansion larger than that of the substrate). That is, upon arranging the intermediate layers in a manner that the coefficients of thermal expansion thereof are inclined, it is allowed to effectively relax the thermal stress due to the difference in the thermal expansion between the surface protection layer and the ceramic substrate, whereby the surface protection layer is effectively prevented from peeling. As a result, the anti-corrosion ceramics of the present invention permit the layers to stably exhibit their properties, and offers improved resistance against the water vapor of high temperatures, lending itself well for use, particularly, as parts for constituting internal combustion engines such as gas turbine engines that operate at high temperatures.